Mine is currently with the Martin Premium Graphite Gray Polygut M605.
I always had the M600 on S1. Tried many other strings, and always ended taking them off within a day and went back to M600.
Until the M605. I didn’t like it at first. In fact hated it. Mostly due to the tactile texture which is kind of rough especially when doing slides. But somehow, I let it settle in for a week or so. Then it grew on me. Once I gotten used to the texture, I felt my S1 sounds better with it than with the M600.
It’s like an enhanced M600 with the same Jangle and loudness. And it comes with better individual note sound. With the M600, I felt it’s a bit 1 dimensional or hollow? But now it’s full. Sorry, I’m bad with the description. Sustain is about the same.
So I guess the folks at Martin know what they are doing when the higher end martin ukes now come with the premium polygut strings out of box.